1. Field of the Invention
The present invention relates to a perpendicular magnetic thin film and a manufacturing process thereof. The perpendicular magnetic thin film is used for the media of a perpendicular magnetic recording method or magneto-optic recording method which enables ultra high density recording and playback in the field of high density memory such as magnetic disks or digital recording for computer memory, video tape recorder, digital recording audio tape recorder, and magneto-optic disks.
2. Description of the Prior Art
Recently, the memory related field tends toward recording with higher density and digital recording. In the field of magnetic recording by means of a magnetic head which accounts for the most part of the memory field, the in-plane magnetic recording method in which the magnetizing direction is in a plane of magnetic recording medium has been used conventionally. However, there is a limit to the improvement of recording density by this method. On the other hand, there emerged a perpendicular magnetic recording method to provide ultra high density recording method, which employs a magnetic recording medium having an axis of easy magnetization in a perpendicular direction to the head travelling surface of the magnetic recording medium. (For example, S. IWASAKI, Y. NAKAYAMA; "An analysis for the magnetization mode for high density magnetic recording." IEEE. Trans. Magn., MAG-13-(5), 1272 (1977)). That is to say, in order to realize this ultra high density recording method, a perpendicular magnetic film is indispensable as a magnetic recording medium material.
On the other hand, active studies have been conventionally continued to develop media which use continuous thin film of magnetic material for the reason that they are more suitable to the ultra high density magnetic recording medium than the coat-type media, and the vacuum vapor deposition method or the sputtering method has been mainly used as a manufacturing process of the continuous thin film of magnetic material. As a material, thin film of Co-Cr alloy in particular, which is easily made into the above-mentioned perpendicular magnetic film, has been studied by a very large number of researchers as a material potentially usable for a medium for the above-mentioned perpendicular magnetic recording. (For example, K. Ouchi: "Co-Cr recording film with perpendicular magnetic anisotropy", IEEE. Trans. Magn. MAG-14(5) 849 (1978)).
However, it was found that many problems are involved with environmental resistance such for example as the corrosion of the magnetic film caused by moisture in air. Experiments have been carried out for improved corrosion resistance by such means as coating the surface of the magnetic film with a non-magnetic oxide layer of different type. However, a new problem has emerged in that the manufacturing process of the magnetic media become complicated. There are many other problems to be solved.
As compared to the above, Fe.sub.3 O.sub.4 (Magnetite) or .gamma.-Fe.sub.2 O.sub.3 (Maghemite) which is the iron oxide having spinel type oxide crystal structure is by itself a magnetic oxide. The .gamma.-Fe.sub.2 O.sub.3 in particular has very extremely excellent environmental resistance and is practically used as raw material magnetic powder for most of the coat-type magnetic recording media such as magnetic tapes and magnetic disks. For this reason, studies have been conducted in an attempt to use these spinel type iron oxides as magnetic films of the magnetic thin film media. Although the continuous magnetic thin film used for media of conventional in-plane magnetic recording method was materialized, the perpendicular magnetic film required for the media of perpendicular magnetic recording method of the above-mentioned ultra high density magnetic recording method has not been materialized.
For the purpose of reference, a typical manufacturing process of the magnetic thin film of the media for the spinel type iron oxide in-plane magnetic recording method will hereafter be described.
Conventionally, the .gamma.-Fe.sub.2 O.sub.3 thin film used to be manufactured by the sputtering method. By means of the reaction sputtering method in which a metallic iron target is used and sputtering is carried out while oxygen is being flowed generally, a thin film of .alpha.-Fe.sub.2 O.sub.3 (alpha iron oxide) having a non-magnetic corundum type crystal structure is manufactured first. The thin film is reduced in a current of hydrogen and caused to be transformed into Fe.sub.3 O.sub.4 having a spinel type crystal structure, which is further caused to be oxidized slightly to finally produce a continuous film of .gamma.-Fe.sub.2 O.sub.3 having the same spinel type crystal structure (for example, J. K. Howard; "Thin films for magnetic recording technology; A review" J. Vac. Sci. Technol. A. 4, 1986).
In the above-mentioned manufacturing of continuous film of .gamma.-Fe.sub.2 O.sub.3 by means of sputtering method, a metallic iron is used as a target material, a continuous film of .alpha.-Fe.sub.2 O.sub.3 is first formed by accomplishing the reaction sputtering in a chamber where a small quantity of oxygen is introduced, the film is reduced to be changed into an Fe.sub.3 O.sub.4 film having different crystal structure, and the Fe.sub.3 O.sub.4 film is further caused to be oxidized slowly to be changed into a continuous film of .gamma.-Fe.sub.2 O.sub.3, thereby a continuous magnetic film of .gamma.-Fe.sub.2 O.sub.3 is produced. Therefore, fine cracks are likely to be generated over the film surface due to a large change in volume caused during a changing process of .alpha.-Fe.sub.2 O.sub.3 .fwdarw.Fe.sub.3 O.sub.4 .fwdarw..gamma.-Fe.sub.2 O.sub.3, resulting in poor film surface quality causing the film to be susceptible to defects. Further, since the film consists of fine a polycrystalline structure, the efficiency of magnetic characteristics is lowered as compared with a single crystal film in which a certain regular crystal plane is in a specific orientation. Furthermore, complicated and numerous processes are involved in manufacturing. That is, there are many problems to be solved.
As a manufacturing process to remove these defects, a method is being studied wherein the powder of spinel type iron oxide is pressed and formed into a target material, so that the spinel type iron oxide thin film is directly obtained by sputtering method (for example, Y, Hoshi, M. Naoe; Telecommunication Society Report, Vol. 85, No. 87, p. 9, 1985 in Japanese). In this process, although a spinel type iron oxide magnetic film of oriented crystal plane can be obtained by selecting a substrate material, the perpendicular magnetic film is not obtained. In addition, this process has a defect such as slow film forming speed and cannot be said to be a manufacturing process suitable for mass production.
On the other hand, an optical recording method which uses a laser beam is available as ultra high density recording method. The magneto-optic recording method is the most prospective rewritable method among the optical recording methods. In the magneto-optic recording method, a laser beam is used to heat a part of a magnetic spin which has previously been oriented in one direction perpendicularly to the magnetic thin film surface of a medium so that the magnetic spin of the laser beam irradiated film surface is magnetized and inverted thereby to record a bit of a signal. The recorded signal is read by a light by utilizing the property of light that the rotational direction of the polarized light plane becomes different depending on the direction of the magnetic spin. Therefore, a perpendicular magnetic film becomes inevitably necessary for the magnetic film of the media of the magneto-optic recording method.
Manganese bismuth (MnBi) alloy was a material of the magneto-optical memory used in the first stage, but had defects such as phase transfer, thermal instability, susceptible to oxidation, difficulty to obtain uniform film of small grain, and large noise of media. Then there emerged a film of RE-TM (rare earth-transition metal) amorphous alloy such as Gd-Co, Tb-Fe and Gd-Fe. These alloy films are normally manufactured by the vacuum deposition method, or sputtering method. The RE-TM alloy features the fact that signal recording with a large signal to noise ratio is achieved by selecting its composition, but it is oxidized, corroded and deteriorated at high temperature and under high humidity because it is a metallic film. This defect is deemed one of the large problems of this method (for example, S. Uchiyama "Magneto-Optical Recording", Surface Science, Vol. 8, No. 2, P. 2, 1987 in Japanese).
The material which is attracting attention as a material to solve this defect is an oxide type perpendicular magnetic film which has good magneto-optical characteristics. This film is a spinel type iron oxide thin film, that is, a cobalt ferrite thin film of which a part of iron ion in the composition is substituted by cobalt ion.
As described previously, a perpendicular magnetic film of Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3 has not been realized yet, but it is possible to manufacture the perpendicular magnetic thin film of cobalt substituted spinel type iron oxide which can be used for magneto-optic recording media. This thin film can be manufactured by a manufacturing process to be described below and is being put to use, but it is currently difficult to obtain a perfect perpendicular magnetic film of low media noise.
Two manufacturing processes are available to produce the above-mentioned perpendicular magnetic film of cobalt substituted spinel type iron oxide for the magneto-optic recording method.
One process is the sputtering method (for example, M. Gomi, T. Yasuhara and M. Abe, "Sputter-Deposition of Co Gr Ferrite Thin Film for Magneto-Optic Memory", Japan Applied Magnetism Society Magn., Vol. 9, No. 2, 133, 1985 in Japanese). As a target, a sintered spinel type iron oxide material containing cobalt is used to manufacture a film under 10.sup.-2 to 10.sup.-3 Torr vacuum and at over 350.degree. C. substrate temperature. In the thus obtained film the direction &lt;111&gt; that is different from the axis of easy magnetization of the cobalt substituted spinel type iron oxide is oriented preferentially. When the produced film is cooled to the room temperature, stress is applied to the film due to the difference in the ratio of thermal expansion between the base substrate and the produced film, causing the magnetic spin of the produced film to be easily directed perpendicularly with respect to the surface of the produced film by the effect of stress-caused magnetic anisotropy; thereby a film having a tendency towards perpendicularly is obtained.
However, because the target is an oxide, such problems still remain that the growth of the sputtered film is not fast as that of the metallic target, it is difficult to obtain a uniform film of large area, all being related to the manufacturing of perpendicular magnetic films for magneto-optic recording media.
The other process is to manufacture a film by means of the solution spray-dry pyrolysis method (for example, J. W. D. Martens & A. B. Voermans: "Cobalt Ferrite Thin Films for Magneto-Optical Recording" IEEE Trans. Magn., MAG-29(5), 1007, (1984)). In this method, a dilute alcohol solution of iron nitrate and cobalt nitrate is used as a raw material, a very thin coated film is produced on a base substrate by using the Spinner method, the coated film is dried at a temperature of about 350.degree. C., similar coated films of greater thickness are further formed again on the same substrate by repeating 50 to 60 cycles of film formation and drying processes, and this thick coated film is burned at a temperature of over 600.degree. C. to obtain a perpendicular magnetic film of cobalt substituted spinel type iron oxide similar to one as described above. This method also utilizes the difference of the thermal expansion ratio between the base substrate and the produced film to realize the film having a tendency to the perpendicular magnetic film by the effect of stress-caused magnetic anisotropy. Furthermore, because this method is capable of providing a film which is preferentially oriented into the &lt;100&gt; direction which is the axis of easy magnetization, this method is excellent in forming a film having a tendency to perpendicular magnetic film through the coating and burning processes, and has the merit of obtaining a uniform film over a large area.
However, because many repeated processes of coating and drying are necessary, a burning temperature as high as 600.degree. C. or over requires materials of excellent heat resistance for the base substrate, and since the film is produced by burning, the size and shape of grains are not uniform, which easily causes the media noise.